BCGS: Carbonatites, Nepheline Syenites & Related Rocks in British Columbia (Chapters 4,5&6) (Pell, 1994)

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BCGS: Carbonatites, Nepheline Syenites & Related Rocks in British Columbia (Chapters 4,5&6) (Pell, 1994)

  1. 1. ULTRABASIC DIATREMES IN NORTHERN BRITISH COLUMBLQ " Only twobreccia pipes have so far been recognized in pearance of a stretched-pebble conglomerate. The northernBritish Columbia north Prince George of (Figure 2); the Os- and central parts of the diatreme havebeen cut by fluorite-pika pipe near Peace Reach,east of Williston Lake and the calcite and fluorite-calcite-pyrite stockwork veins contain-a small diatremein the Kecbika Riverarea of the Cassiar ing minor amounts of galena and molybdenite. SimilarMountains, westof the Rocky Mountain Trench. Both are breccias (minus thephlogopite) arepresent the E d River inhosted by middle Paleozoic carbonate rocks are associ- and -White River area of the southern Rocky Mountains (Yell,ated with carbonatite/alkaline rock complexes. The dia- 1987).treme in the Kechika River area is the only one documentedwest of the Rocky Mountain Trench. Associated dikes are quite common periphcal to the main diatreme and on ridges to the north (Figures 21 and theTHE KECHIKA RIVER DIATREME AND 22). They crosscut both the carbonate hostrock!; and the mottled phyllites. The dikes, in general, are extremely wellRELATED ROCKS (94L/12,13) foliated and average 1 to 2 metres in thickness. They are A suite of alkaline igneous r 9 k s in the Kechika Ranges similar in composition and appearance the matrix of the toof the Cassiar Mountains is intermittently exposed in a main diatreme, comprised predominantly iron and mag- ofnorthwest-trending zone excess of 20 kilometres long, the incentre of which is approximately latitnde 58"42north and TABLE 13longitude 127"3Owest.Tracbytes,syenites,malignites,car- GEOCHEMISTRY OF SELECTED DIATREMEbonatites and related tuffs and agglomeratesare present in BRECCIAS AND RELATED DIKESAND T JFFS , Ithissuite(seeChapter2);adiatremebrecciapipeandrelated KECHIKA AREAdikes and tuffs are also exposed in this area. The igneous - ~~~~~~ ~. "rocks are hosted by middle Paleozoic (Silurian?) carbonate Diatreme brecciastrata and have been deformed and metamorphosed to 13.34 35.68 36.75 41.46 34.35 37.04 19.82greenschist facies. 0.85 1.14 1.18 1.12 0.74 0.81 0.95 A1203 6.07 10.07 6.32 5.35 10.35 10.42 A complex diatreme containing a numbes of breccia 9.78 9.48 5.96 7.13 6.92 7.4: 6.81phases, related tuffs and breccia dikes crops out near the 0.14 0.12 0.38 0.39 0.48 0.41. 0.26centre of the belt of alkaline igneous rocks(Figures 21 and 5.1.1 3.65 15.11 8.45 10.68 10.01 9.3;22). These rocks weather greenish to rusty orange and silver 9.55 5.44 12.55 9.89 11.27 20.21 11.7are weakly to extremely well foliated. The main diatreme is 0.06 2.13 2.10 2.74 2.96 0.2:; 0.72exposed in acreek at approximately 1560 metres elevation; 4.80 1.71 7.74 6.04 4.20 4.0:: 3.76dikes and tuffs are present on the slopes and ridges to the 16.87 14.79 14.2215.88 23.31 13.05 28.4:north and west of the diatreme, at elevations of up to 2230 11 L0.24 0.19 0.26 0.21 1.3: 218metres. Exposure in the area is moderate to excellent; buck- 6.68 99.30 98.71 97.51; 99.11 98.04 95.49brush andscattered trees are present in the valley bottoms,while the upperslopes are barren. Access to the area is by 689210600 270514 71 1 1G2 1100 1108 510 720 - 32r1 641helicopter from Watson Lake, Yukon or Dease Lake, B.C., - 82 - 269 2 1 56150 and 160 kilometres distant, respectively. 390 95314 230 320 105 127 295 212 790 610 708 81 92 420LITHOLOGY Ba 970 179 500 430 412 411 174 Zr 105 106 225 440 181 56 175 The main diatreme (Figure 22) comprisesvery inhomo- Nb 9 60 20 <5 92 43,3 80geneous, heterolithic tuffisitic breccias with roundedto an- 32 23 32 55 50 3885 46gular xenoliths up to 7 centimetres in diameter. Quartzite 15 341 199 44026 317 360and carbonate rock fragments dominate xenolith popu- the 33 35270 560 423 440 510 - 47 204 -lation; some autoliths, rare syenite fragments and someblack argillite clasts were also noted. Quartz xenocrysts,rare chrome spinels, juvenile and vesiculated glass lapilli,and crystal fragments (predominantly potassium feldspar : ;1 Nd Ta < s < 3< 5 < 5 201 75 9, 23 5 areand minor phlogopite) also present. The breccia matrixconsists of carbonate minerals, potassium feldspars, minor 161muscoviteandlocally.chromemicas.Inp1acesnearitsoutercontacts, the breccia is intensely deformed andhas the ap-Bulletin 88 71
  2. 2. Brifish Columbianesium-rich carbonate minerals, feldspars, muscovite andserpentine. Some quartz and apatite may also be present.The dikes locally contain chrome spinels, small lithic frag-ments and fragments of devitrified glass. Some contain orchrome-green (chrome mica) dark green (chlorite and bi-otite) elliptical patches which probably represent shearedand altered fragments or crystals. One dark green weather- rocking dike contains abundant small fragments and alteredolivine macrocrysts. Tuffs outcrop on ridges near the centre of the property,immediatelynorth of the main diatreme and the north end atof the property, south of Boreal Lake (Figures 21 and 22).These pyroclastic rocks are rusty orange to silver-greenweathering with a pale green fresh surfaces, very similar inappearanceto some of the dikes. They are conformable withthe host carbonate succession and are interbedded withbrown, blocky weathering agglomerates and aplitictrachytes. Chrome spinels are present locally. In thin sec-tion, these rocks are seen to contain plagioclase laths,siderite spots and altered, six-sided crystals (clinopy-roxenes) in a fine-grained matrix of carbonate, sericite or IC,feldspar and oiaqnes. These rocks be the extrusi! ~~~~ ~ ~~ ~~~ ~~may~ ~- @ 0 didreme dike X + tuff ultramafic Figure 58. Major element ternary plots, Kechika diatrelre and 00 .0 5.00 15.00 10.00 20.00 25.00 related rocks. 0 0 Kechiko dialreme 25.00 20.00 0 diatreme X dike + tuff 1500.00 4 * and dikes Kechika tuffs V 15.00 baranifiie tephriie alkollne larnprophyre 5.00 0.00 m 30.00 20.00 40.00 50.00 60.00 70.00 S I 0L 7 C r Darn gure57.Majorelementdiscriminantplots,Kechikadiafremear Figure 59. "Average" compositionsfrom Wederhi.1and lated dikes and tuffs. Maramatsu, 1979.Nivs Crplot, Kechikadiatreme, dikes an(ituffs.72 Geological Survey llranch
  3. 3. Minisrry of Energy, Mines and Petmleun: Resources " Based on modal mineralogy (olivine, clinopyroxene, plagioclase and/or potassium feldspar and minor phlo- gopite) it is difficult to classify these r o c k they showsome CHONDRITE-NORMA1 17Fn REE PLOT similarities to alkaline basalts (basanites). RELATED GEOCHEMISTRY Major element analyses of samples fromth,: Kechika 0 diatreme diatreme and related dikes and tuffs (Table 13) in(licatethat X dike these rocks are relatively low in silica and aluniinum and + tuff moderately enriched calcium and in magnesium. On a K20- MgO discriminant plot samples from Kechikt pipe and the related rocks fall predominantly within or near the leucitite field (Figure 57a), while on an alkali-silica discriminant plot they fall between the alkaline lamprophyre and alnoite .field (Figure 57b). On the ternary Fe-Al-Mg plot, samples fall within the melilitite (alkaline lamprophyre) field (Figure 58a). On an AFM plot,samples plot closer to the: base (the A-M side) than alnoites or typical basalts (Fignrt: 58b); the diatreme samples are similar, in this respect, to samples of other rock types in the Kechika area (see Figure 24b). Based on major elementchemistry, it is difficult to c1ar:sify these rocks although they do show some chemicalaffinity to al- kaline lamprophyres (nephelinites) or leucitites. Alteration may have affected major element chemistry enor.gh to pre- clude definitive classification. Diatreme anddike samples show afair rangc: of nickel and chrome values; some are moderately enriched, contain- 1 ing up to 0.11% chrome and 0.07% nickel. Tht:y contain Lo Ce Pr Nd Srn Eu Tb Dy Ho Trn Yb Li higher concentrations of these two elements than do there- Rare-earthElements latedtuffs,inwhichtheigneouscomponenthasbeendiluted (Figure 59). Diatreme and dike samples also moderately areFigure 60. Chondrite-normalized REE plot - K e c W diatremes enriched in fluorine, containing between 0.18 and 1.9% (Ta-and related dikes and tuffs. ble 13). They contain low to moderate amounts ::areearths of ~~ Plate 31. Dolostone clastwith reaction rim, Ospika pipe. ~-Bulletin 88 73
  4. 4. (up to 510 ppm La) and, on chondrite-normalizedrare-earth OSPIKA PIPE (94B/5)plots, generally display shallow, negatively sloping curves The Ospika pipe is a small diatreme located on(Figure 60) which are indicative of a low moderate degree to Comincos Aley claims (latitude 56"27N, longitudeof light rare-earth enrichment. The diatreme and related 123"45W) approximately 140kilometres north-nortttwestrocks are anomalously enriched rubidium and have in very of Mackenzie, on the east side of Williston Lake bemeen the Peace Reach and Ospika River. Access to the area is bylow total strontium:rubidium ratios that average around 2: 1 helicopter from Mackenzie.(Table 13). The diatreme crops outon forested slopes at approxi- mately 1550 metres elevation (Figure 3). It is only 2, fewGEOCHRONOLOGY hundred metres from the large Aley carbnatite coniplex (see Chapter 2), butthe relationships between the carlmna- No radiometric dating has been done on alkaline rocks tite and the diatreme are unclear.from the Kechika area. Field relationships suggest that theyare similar in age to the host strata, Silurian or slightly LITHOLOGYyounger. The Ospika pipeis a composite diatreme(roughly 20 by 50 metres inarea) containing at least five distinct brxcia TABLE 14 and massive phases and intruding Ordovician carbonms of CHEMICAL ANALYSIS - OSPIKAPIPE the Skoki Formation. is massive to foliated and red-brown It ~ ~ ~ ~~~ weatheringinoutcrop,withfluoritepresentnearthemargins 1 2 3 4 75 6 27.9032.0926.660.99 24.1830.5928.65Ti02 2.76 2.15 2.76 1.67 2.25 2.58 0.38Ai203 5.43 4.77 6.25 5.01 5.19 5.61 0.25Fe203T 9.47 7.94 9.25 9.56 7.61 8.67 5.21 0.20 0.19 0.17 0.22 0.16 0.17 0.55 12.72 10.04 14.65 10.62 12.47 14.07 14.69 15.63 17.26 13.40 17.20 17.52 14.84 27.53 0.01 1.23 0.95 2.29 1.55 1.85 0.00 1.43 4.01 5.55 3.80 4.01 4.02 0.12 10.00- 11.51 24.79 10.83 14.32 18.07 17.05 41.59 1.12 0.84 0.09 1.80 0.99 0.08 0.28 0.12 0.16 0.01 0.80 0.41 I %.25 97.67 96.93 97.25 97.50 99.2691.81 9 5;0 . 5.00- 230 190 360 220 290 40 330 455 331 481 185 409 232 387 co 48 24 46 39 40 10 47 Rb 48 42 160 115 148 <8 118 Sr I 514 1257 1134 1650 1448 837 1601 0.00 Ba 788 1163 1248 1741 1737 1290 20138 Zr 300 53 263 682 261 303 4 Nb 241 625 180 245 147 168 226 Y 36 72 32 51 26 26 34 La 183 177 124 170 96 199 3858 25.00 Ce 322 332 217 369 85 166 6298 Nd -; Yb <-2 2 <-4 <-7 0 <-1 6 20.00 sc 33.2 29 32.1 29 32.6 28.8 31.5 Ta 13 15 10 16 6 a 16 Th 90 36 21 40 16 1731 17 15.00 +phonolite u 16 13 16 19 5 16 8 V 317 398 353 261 342 253320 4cu 109 74 73 103 112 119 _______ 80 10.00 from Alno frachybaaalf 5.00 barall +andesite I klrnberlife 0.00 rn 902 Figure 61. Major element discriminant plots, Ospika pipe.74 Geological Survey Branch
  5. 5. Ministry of Energy, Mines and Petroleum llesoumeson the pipe. The phases are differentiated by size and per- Phlogopite dominates the macrocryst assemblages,centage of fragments of sedimentay rock, macrocrysts and comprising 5 to 20% of the rock, with titaniferous augite,pelletal lapilli. Contacts between phases may be sharp or rare altered olivine and bright green diopside also presentgradational. Locally, narrow dolomitic dikes crosscut dia- locally. Phlogopites range from few millimetres to 3 cen- a timetres in size, augites from a few millimetres tc. 2 centi-treme breccias. metres. The phlogopites are orange in colour and have The breccias contain 2 to 25% subangnlar to snb- normal pleochroism. In some samples, they have thin,rounded fragments of sedimentary rock. These range from bleached rims; in others, the rims are darker than the corea fewmillimetres to 50 centimetres across, with most in the of the grains.2 to IO-centimetre range. Larger fragments are dolomitic The matrix to most phases is fine grained and lightwith prominent reaction rims (Plate 31). Rare cognate xeno- green-grey in colour.It is a g o d igneous or magmatic ma-liths are present, but no exotic xenoliths were found. One trix, consisting of fine-grained dolomite and felled plrlo-distinct phase is composed of abundant small pelletal lapilli gopite, chlorite, amphibole with or without talc. It contains abundant fine-grained opaque oxides and, in some places,(50-60%).macrocryptic phlogopite (5-10%) and small frag- pyrite.ments of sedimentary rock(less than 10%) in a fine to me- Clast andmacrocryst-rich brecciadikes, 50centimetresdium-grained carbonate matrix. wide, crop out onridges 0.5 to 1 kilometre away fromthe main breccia pipe. These dikes do not appear be continu- to ous with the diatreme at surface, but have very similar clast and macrocryst populations. Locally, the matrix the of dikes is considerably morecalcareous than that of the diatreme. Both thedikes and mainpipe havesuffered some (degreeof alteration. Bluepleochroic sodium amphiboleis ukdquitous, often rimming other phases. The Ospika pipe and related dikes may be classified, on the basis of petrography, as ultramafic lamprophyresusing the criteria given byRock (1986)and, more speci:ically, as aillikites. These arerelatively common ultramafic lampro- phyres that are often associated with carbonatites. They are similar to alnoites, but lack goodevidence of meltlite. GEOCHEMISTRY Major element analyses of samples from the Ospika pipe and relateddike rocks (Table 14) indicate that the pipe is low in silica and high in calcium, magnesium andiron. On a KzO-MgO discriminant diagram (Figure 61a:r analyses group in an area which is overlappedby the leucitii e, olivine melilitite and alkaline lamprophyre fields. It should be noted, however,that on this diagram, alnoites from Alno fall ~~ ~ ~~~~~~~~ ~ __ ~ ~~ 0 Ospika pipe 1500.00 Average basalt Average kirnbtrliles 0.00 1000.00 500.00 1500.00 Cr ppm _- Figure 62. Major element ternary plots, Ospika pipe Figure 63. Ni-Cr plot, Ospika pipe.
  6. 6. ~ ~ ~~ ~~~~~~within the alkaline lamprophyre field, which suggests that The geochemical data do not conflict with thc pet-the ultramafic lamprophyre field should be extended; this rographic classification of the pipe as an ailliki:e, orplot is not strictly adhered to in attempting a chemical clas- melilite-free variety of alnoite, which, according to Rocksification. Analyses from the Ospika pipe plot relatively (1986) is a member ofthe ultramafic lamprophyre family.close to the type alnoite. The sameis true on an alkali-silicaplot (Figure 61b). Ternary AFM and MgO-AIz03-Fe~03plots (Figure 62) show that analyses from the Ospika pipe GEOCHRONOLOGYplot in or near the alonite and/or aillikite (a melilite-free Rubidium-strontium isotopic studies on mica sep,uatesalnoite) fields. from the Ospika pipe give an age of 334f7 Ma. Potas-’1111- ,1 1 The Ospika pipe has a restricted range of nickel and argon studies on the same sample yielded 323f10 Ma (Ap-chrome values and contains relatively low concentrations ofthese elements, similar to the ‘average’ nephelinite (Figure pendix 2). These data suggest that the pipe very similar in is63). It is somewhat enriched titanium, barium,strontium in age to the Aley carbonatite complex, which it flanks. Theand niobium, and has fairly restricted range of total stron- a temporal and spatial association between carbonatites andtiummbidinm ratios, averaging around 13:l (Table 14). alnoites or aillikites is well documented (Rock,1986:..76 Geologicul Survey B m c h
  7. 7. Minisiry o Energy, Mines and Peiroleum Resourres f " ULTRABASIC DIATREMES IN THlE GOLDEN - COLUMBIA ICEFIELDS AR:EA Numerous diatremes are located along the Alberta - suggests that these rocks are neither kimberlites nor lam-British Columbia border @2N, 83C) between 50 and 90 proites, the twolithologies currently known to contain eco-kilometres north of Golden (Figures 2 and 64).The terrain nomic concentrations of diamonds.in the area is rugged and the diatremes outcrop elevations atof 2200 to 3000 metres. In all cases access is by helicopter. BUSH RIVER AREA (LARRY CLAIMS)Most of the diatremes are hosted by Upper Cambrian car- (83CP)bonate rocksand, in most cases, consanguineousdikes are Near the headwaters of the Bush River :latitudealso present. 52"0420"N, longitude 1172350W Figure 64), I suite of Microdiamonds havereportedly been recovered from dikes and small diatremes intrudes Upper Cambrianstrata.heavy mineralseparates taken fromtwo of the pipes inthis Three diatremes were examinedin this study (Figure 6 3 ,swarm (Northcote, 1983a. b; Dummett et al., 1985; George revealing two breccia types. The breccias appear ,quitedif-CrossNewsLetter,Jan.23,1990).Preliminaryinvestigation ferent in the field, but the division is somewhat arbitrary. A Bush Riverpipes dikes and B Lens ountain ipe M p C Mons reek rea C a LEGEND D V a l e n c i e n n e sR i v e ra r e a Foliated breccia phose with pelletal lapilli E HP p i p e Rusty weathering quartz xenocrystic bre:cia Fine grained massive phose dike Mocrocryrt rich dikes (micas, pyroxenes, etc.)Figure 64.General geology and diatreme locations the Golden in Macrocryst rich dikes with breccia core:$- Columbia Icefields area. indicates diatremes or dike swarms.GeologymodifiedfromWheeler(1962)andPrice(1967a,1967b). _-For legend see Figure 73. Figure 65. Diatreme brecciasand dikes, Bush River m a (83CL3).Bulletin 88 77
  8. 8. Plate 32. Rusty weathering, clast-supported megabreccia, Bush River aea.78 Geological Survey Branch
  9. 9. ~ .~ . _ r: : ..... .. ~. l -. .. ~ ~ ~ ~ .". , Minisrry o Energy, Mines and Pefmleum Resources fPlate 34. Limestone-cored armoured xenolith in diatreme breccia,Bush River area. Plate 36. Boulder from adike, Bush River area. Dike has a brcccia core containing abundant fragments of sedimentary ~.ockand a finer grained, macrocryst rich rim.Plate 35.Altered micamacrocryst in diabeme breccia, Bush River Plate 37. Laminated (flow-banded) margin a fine-grained dike, ofarea. Bush River area.Bulletin 88 79
  10. 10. The pipes are massive; onlythe southeastern diatreme hasa margin that is foliated, with the foliation subparallel to thecontacts (Pell, 1987; Ijewliw and Schulze, 1989). The northeastern pipe comprises rusty orange weather-ing, clast-dominated megabreccia (Plate 32). The clastma-trix ratio is approximately 39. Over 99% of the clasts are ofsubrounded to subangular fragments the hosting carbon-ate lithologies, ranging in size from 1 to 75 centimetres, withan average size of 10 to 40 centimetres. Altered granitoidsand, less commonly, gabbroic rocks make the balance of upthe xenolith population. The matrix is predominantly car-bonate and quartz-sand grains. The second type breccia, exhibited by the central and ofsouthern pipes, is also clast dominated andgenerally mas-sive, but is rusty to dark green weathering (Plate 33) ratherthan orange in colour.The clastmatrixratio is greater thanin the first breccia type and clast population more the varied.Approximately 50% of the clasts are subangular fragmentsof sedimentary rock, carbonates, shales and some ortho-quartzites. Two to five percent of the xenoliths consist ofgranitic material; these fragments may be either rounded orangular and are in the 5 to 15-centimetre size range.Rounded, 8 to 25-centitnetre fragments of altered igneous-looking material comprise 10 to 20% of the xenoliths. These Plate 39. Fragments of sedimentary rocks in a buff-weathering,clasts consist of coarse, randomly oriented carbonategrains, quartz xenocryst-rich breccia, Mons Creek area. Plate 40. Photomicrograph of quartzxenocryst-richbn:ccia,Plate 38. Photomicrograph of an altered pyroxene crystai in a similar to that shown in previous photograph.Quartz xenwrystsmatrix containing abundant altered mica; dike, Bush River area. are enclosed in a matrix of carbonate, chlorite and iron-o:ddes.Long dimensionof photograph is2.5 mm. Long dimension of photograph is2.5 m .m80 Geological Survey Branch
  11. 11. Ministry o Energs Minesand Petroleum Resources fFigure 66. Sketch showing distribution diatreme related rocks the lack claims Lens Mountain area; sketch drawn from photog,raph of ontaken by C. Fipke. View is of the ridge southeast of Lens Mountain, facing northeast.chrome mica and opaqueoxides. Spectrographic analyses surfaces are generally a dull greenish grey colour. They haveindicate high silica content; these clasts are possibly altered coarse xenolith andlor macrocryst-rich cores and finersyenites. Many of the rounded xenoliths are armoured, or grained margins (Plate 36). Contacts withiuthe (dikes maymantled bya rim fine-grained mica-rich igneous of material be gradational or distinct and often the margins have asimilar to the breccia matrix. An additional 5 to 10% of the strongly flow-banded texture (Plate 37). In thin swtion, thebreccia fragments arecognate xenoliths. The remainderof dikes are porphyrytic with altered macrocrysts, phenoclyststhe clast population is made upof spherical structure (also and glomerocrystsof olivine, pyroxene (Plate 33) and bi-referred to as accretionary lapilli or globular segregations) otite. The olivine crystals, some of which contain inclusionsranging from a few millimetres to 3 centimetres in size and of red-brown spinel, are altered to calcite, serptmtine andfrequently cored by small fine-grained limestone fragments talc. The groundmass consists of a network altered of biotite(Plate 34). Armoured xenoliths and accretionary lapilli are with dusty secondaq calcite, minor serpentine, soinels andfeatures common to pyroclastic rocks (Fisher and opaques (Ijewliw and Schulze, 1989).Schmincke, 1984). Silvery, altered mica macrocrysts, up to Based on modal mineralogy, including the pseudomor-3 centimetres in diameter, are abundant (Plate 35); they phed phases (olivine, pyroxene, biotite, plagiodase). thewere, most likely, originally biotites. The matrix the brec- of dikes and diatremes the Bush River can be in area classifiedcia consists of chlorite>calcitequartz>trace apatite. In thin as lamprophyres of either alkaline or calcalka1ir.e affinity.section, it is seen to comprise 25% euhedral to suhhedral The best designation appears to be as olivine-kzrsantites,olivine crystals that have been pseudomorphedbyeither ser- which are part of the calcalkaline lamprophyre familypentine, or calcite and quartz with magnetite rims, cloudy, or (Ijewliw and Schulze, 1989) biotite-camptonites, whichbrown plagioclase, biotite, euhedral calcite and trace are alkaline lamprophyres.amounts of magnetite and apatite phenocrysts in a fine-grained aggregate of dusty carbonate, quartz, serpentine, LENS MOUNTAIN AND MONS CRJ3EKmagnetite, chlorite, felsic microlites and unidentifiable ma- AREAS (JACK AND MIKE CLAIMS)terial (Ijewliw and Schulze, 1989). (82N/14,15) Numerous subparallel dikes are present in the BushRiver area, some of which crosscut the diatreme breccias; At both Lens Mountain (latitude 5 l”54’30”N. longitudethey range in length from 50 to 600 metres and width from in 117°07‘30”W) and Mons Creek (latitude 51”49’:;O”N, lon-0.5 to 2.5 metres. Bifurcation and remerging occurs along gitude 117°00’30”W; Figure 64)the dominant intrusive li-the length of some dikes and narrow, fine-grained apophyses thology consists of a buff-weathering, weakly foliatedare common. Both homogeneous and zoned dikes are seen breccia with a low clastmatrix ratio (approximalely 1:3 or(Figure 65). The homogeneous dikes are fine grained and 1:4). The ‘Jackdiamond‘(Northcote,1983b)was recoveredmedium to dark green in colour. They may contain up to from this lithology and two additional microdiamonds have10% small, silvery, mica phenocrysts and minor amounts of recently been recovered from core (George C ross News drillspinel or other opaque oxides. A dike with unaltered phlo- Letter, Jan.23,1990, p. 2).Clasts are small subaugnlarfi‘ag-gopite megacrysts was observed one locality.The zoned in ments of sedimentary rock, predominantly carbpnates, indikes are rusty to dull green on weathered surfaces and fresh the 2-millimetre to 2-centimetre size range (Plate 39). The __Buliefin 88 81
  12. 12. British Columbiamatrix is pale green to buff incolour and consists ofabun- dark red and have dark greyfresh surfaces. They consist ofdant rounded quartz grains (xenocrysts), carbonate, chlorite subangular clasts of limestone and relict phenocrysts in aand iron oxides (Plate 40). Relict lapilli, with a preferred carbonate matrix. The matrix consists of 15%phenocrystsorientation, have also been observed(Ijewliw and Schulze, that are entirely pseudomorphed by fine-grained quartz1989). At Mons Creek,this breccia grades into a clast-poor andor calcite. Some of the phenocrystsretain traces of sim-(5 to 10% clasts) green breccia with a foliated matrix con- ple twinning, with a morphology suggestive of sana4line.taining carbonate, chlorite and talc(?) or serpentine. In both Altered crystals of titanamphibole or sphene are also pre-areas, the enclosing sedimentary strata are steeply dipping; sent; they have been replaced calcite but retain a rimof bythe breccias, however, display a weakly developed subhori- inclusions of very fine grained sphene.The groundmrss iszontal to shallow-dipping planar fabric. At Lens Mountain, extremely fined grained and contains calcite patchesa shallow-dipping layer of boulder breccia is enclosed in the (Ijewliw and Schulze,1989).sandy breccias. At Mons Creek, a small, light green, strongly foliated, It has been proposed (C.E. Fipke, personal communi- tine-grained breccia crops out the north of the main sandy tocation, 1987) that these breccias are crater-infill material. diatreme. It contains fragments of sedimentary rock lessAlternatively, they may be intrusive, formed through than 1 centimetre across and opaque oxides a matrix con- influidizing of sediments by introdnction of volatiles explo- sisting of dolomite>quartz2pyrophyllite~inor chlorite,sively exsolved fromrising and vesicnlating magmas. The calcite, muscovite and trace apatite. In thin section, lhesesubhorizontal fabric locally displayed in the breccias would rocks have a porphyrytic texture, but arehighly altered. Cal-have originally been steeply dipping (prior to deformation), cite replaces some phenocrysts, which may have been oli-which would favour the latter hypothesis. vine, manyof which have red-brown spinel inclusions. The original composition of other psendomorphed phenocrysts At Lens Mountain,two additional small breccia pipes is undeterminable (Ijewliw and Schulze, 1989).or dikes and light green, clast-free aphanitic rock are alsopresent (Figure 66). The breccias are very coarse, weather At Mons Creek, one small, altered dike cutting the main diatreme and a second parallel dike outside the diatreme ~~. ~ LEGEND @Green foliated breccia phase @Quartz xenocrystic breccia a Massive diatremephase a Fine-graineddikes Dikes with pyroxene and olivine mocrocrvrtr Dikes with mica rnocrocrysts pyroxene olivine and /LI Host limestone Plate 41. Serpentinizedolivinemacrocrysts in a fine-grained, Figure 67. Diatreme breccias and dikes, Valenciennes River. massive diatreme phase, Valenciennes River area.82 Geological 6ranch
  13. 13. Ministry o Eneqy, Mines and Perroleunr Resources fwere observed. Oneunaltered porphyrytic dike and abun-dant unaltered float of porphyrytic dike-rock are presentelsewhere onthe property. The altered dikes contain quartzaggregates replacing a lath-shaped, twinned mineral thatmay have been feldspar, some minute plagioclase grains(An25) that are partly replaced by calcite, and a phyllosili-cate mineral, partially replaced by calcite, with sphene in-clusions and rims in a fine-grained groundmass ofcarbonate, chlorite and minor quartz and pyrite (Ijewliw andSchulze, 1989). The unaltered dike material contains 5%primary phenocrystic clinopyroxene,commonly augite ordiopsidic augite with pinkish brown titaniferrous rims.Some grains also have titanium-rich cores, and an iuterme-diate, nonpleochroic zone. Other phenocrysts present in-clude biotite, some amphiboles and olivine, completelypseudomorphed by calcite or chlorite, with red-brownspinel inclusions. The matrixcontains microphenocrystsofclinopyroxene, biotite, sphene and plagioclase (Anzs) in agroundmass of carbonate, chlorite, interstitial quartz, fine-grained serpentine and opaqueoxides. The main diatremes in the Lens Mountain and MonsCreek areas have only minor igneous components duetointense sedimentary rock contamination and difficult to areclassify. The dikes at Mons Creek, however, have a pre-served mineralogy (titanaugite, plagioclase, biotite, ampbi-bole, olivine) which allows classification as alkaline Plate 42. Fine-grained boudinageddike subparallel to tedding inlamprophyres, or more specifically, biotite camptonites buff-coloured carbonates. Diatreme breccia in the right(Ijewliw and Schulze, 1989). background. Valenciennes River area.VALENCIENNES RIVER PIPES(MARK CLAIMS) (82N/15) Four or more diatremes and numerous dikes intrudeUpper Cambrian rocks near headwater of Valenciennes theRiver (latitude 51"470O"N, longitude 116"5800W, Fig-ures 64 and 67). Two distinctly different diatreme types arepresent. The first are rusty brown to pale green weathering,weakly to well-foliated, composite pipes with both massiveand breccia phases. Two such diatremes are exposed at thesouthern end of the area examined. Serpentinized olivinemacrucrysts (Plate 41). coarse nonmagnetic oxides(greenspinels) and altered spinel peridotite xenoliths are presentin some phases. Typical breccias contain 30 to 40% clasts,mostofwhicharesmall(l-5cm,withamodeofZcm),withrare xenoliths up to 15 centimetres across. Limestone,dolostone, shale and minorquartzite comprise the majorityof the breccia fragments. Rare oxide macrucrysts may bepresent. The matrix typically light green to grey in colour isand contains calcite and/or dolomite, quartz, chlorite, mus-covite, and traces of pyrite, apatite, talc and clay minerals.Bright green mica is commonly seen in band sample(chrome-rich muscovite?). Chromite and ilmenite havebeen identified in heavy mineral separates and a mi-crodiamond is reported to have been recovered from thelargest diatreme (Northcote, 1983a). Associated dike rocks fine to medium grained, rusty areto dark green weathering, with a light greenish grey tome-dium greenfresh surfaces. They occurin a number of ori- Plate 43. Altered phenocrysts (zonedpyroxenesfolivinehica)inentations, varying from almost concordant discordant to to a dike, Valenciennes River area. "be degree of alteration in thisbedding. Dikes which are nearly parallel to bedding and sample is typical. Long dimension of photograph is2.5 m . the m "Bulletin 88 83
  14. 14. cleavage are strongly baudinaged (Plate 42). Some of thedikes are porphyritic and all are altered. The phenoclyst as-semblage, so far as can be recognized, consists of sieve-tex-tured olivine pseudomorphs, altered euhedralclinopyroxene with relict zoning, mica and rare spinels. Pla-gioclase phenocrysts have been observed (Ijewliw and alsoSchulze, 1989). In some phases, olivine appears to he moreabundant than pyroxene (olivine+pyroxenehica) and inothers pyroxene is far more abundant (py-roxenekolivinehica). Some dikes are predominantly mi-caceous. It is difficult to accurately estimate proportions ofphenocrystsas they are altered and only morphology canbeused plate 43). The groundmass generally consists of veryfine grained carbonate, chlorite, serpentine and altered bi-otite (Ijewliw and Schulze, 1989). Oxides are also a com-mon groundmass constituent. The dikes are generallyperipheral to the diatremes, but locally crosscut them. The second type breccia is present in the diatremes ofin the northern part of the area examined (Figure 67). Theyare brown weathering and moderately well foliated with an-gular to subangular fragments sedimentary rock set in a ofmatrix ofquartz grains, chlorite and carbonate. Clasts aver-age 1 to 5 centimetres across, with some up 20 centime- totres. The clastmatrix ratio is 2:3. These diatremes aresimilar to the orange-weathering pipe the Bush River area in(the first type described). Intense alteration makes petrological classification dif- Figure 68. Geology of the HPpipe, south the Campbell Ice field. of Details on breccia and dike phasessupplied in Table 15. See textficult. The rocks in the Valenciennes River area, as evi- for geographic coordinates.denced by phenocryst assemblages in dikes (olivine,clinopyroxene,mica, plagioclase) also bear astrong resem- in the Golden - Columbia Icefields area (Figure 64).It isblance to those in the Bush Riverarea and probablybelong located approximately 50 kilometres due north of Goldento the calcalkaline or alkaline lamprophyre clans. The best and is exposed at an elevation of 2400 metres, near the toedesignation appears to be as olivine kersantites, which arepart of the calcalkaline lamprophyre family alternatively, or, of the Campbell Icefield. The pipe small, covering an area isbiotite camptonites, which are alkaline lamprophyres. of only 40 by 70 metres; however, it is exposed in a flat, recently deglaciated basin which offers nearly 100% expo-THE HP PIPE (82N/10) sure and is therefore ideal for study. The pipe has sharp, steeply dipping contacts with the T h e H P p i p e ( l a t i t u d e 5l04l3O"N, longitude horizontal to shallow-dipping grey Cambrian limestone116"57W) is the most southerly diatreme so farrecognized beds which it intrudes (Figure 68). It is a composite dia- TABLE 15 FELD CHARACTERISTICSOF HP PIPE BRECCIA PHASES84 Geological Survey Branch
  15. 15. Plate 46. Optically zonedandraditegarnets, HP pipe. Long dimension of thephotomicrographis 2.5 mm. C!arbonatePIate 44. Sharp contact between breccia phases & 3, HPpipe. (82 ) segregation forms matrix.Plate 45. Large gabbroic xenolith a strongly foliated breccia in (BI Plate 47. Biotite macrocryst coring spherical smclure, HP pipe.phase), HP pipe. Long dimensionof the photomicrographis I m . m
  16. 16. TABLE 16 - CHEMICAL ANALYSES GOLDEN AREA DIATREMES - Si02 TI02 A1203 Fe203 0.14 0.13 0.18 0.06 0.17 0.12 0.13 0.18 0.18 0.06 0.12 0.12 0.06 0.16 13.21 11.79 8.67 5.18 9.69 14.67 11.03 12.27 12.79 6.16 6.51 9.48 5.02 7.25 19.92 20.39 17.41 17.74 16.15 13.28 9.48 16.11 13.36 14.51 11.28 12.36 15.61 12.77 0.29 0.27 1.65 1.87 0.25 0.06 0.42 1.32 1.60 0.06 2.73 0.29 0.01 0.18 0.06 4.131 1.01 4.18 4.47 0.01 1.361 0.93 4.031 0.53 0.21 0.70 3.70 0.601 6.44 10.70 7.27 16.29 19.88 16.02 14.05 7.71 5.84 14.50 11.73 14.08 16.11 13.90 P205 0.58 0.61 0.95 0.74 0.81 0.77 0.47 0.88 0.65 0.42 0.11 0.04 0.09 0.07 0.79 0.11 0.16 0.08 0.07 0.08 0.20 0.04 0.31 0.02 Tora, 98.45 97.63 97.54 98.951,401 98.66 98.89 99.43 98.99 98.93 99.02 98.93 98.88 98.21 99.63 0.051 0,581 0.91~ 240 210 140 150 330 200 200 270 230 200 270 230 200 786 904 313 335 301 805 386 493 527 849 399 589 744 387 1101 co 40 40 38 30 42 55 43 45 4 j 48 38 36 56 40 137 121 1301 40 17 8112 3 24 39 < <8 12 85 5 SI 99s lizi lis51 579 881 528 1191 1410 640 1026 821 781 513 595 Ba 1933 1868 3131; 390 202 3791 1975 514 2136 360 91 227 322 589 Zr 156 Nb Y 17 191 244 175 24 90 34 31 78 261 :z 188 361 172 294 20 345 301 22 22 26 25 29 229 265 160 262 200 204 112 198 312 140 229 338 24 24 26 96 111 18 La 92 121 91 78 197 120 119 162 165 145 123 Ce 181 169 193 208 149171 321 206 247 267 29 1 278 220 Nd 4491 Yb 1 <-4 3 <-2 1 0 0 0 <-2 c- 1 33 26.1 31.3 34.328.2 35 25 28.8 31.1 25.1 6 17 18 6 10 9 3 11 11 31 12 14 15 10 21 6 7 . A1 .. 4 _. 17" 7X _. 14 0 7 .^ 23 3L I 33 16 hI 221 322 232 305 206 306 339 356 189278 67 49 11057 80 65 69 109 7 83 73 215 30 _"86 Geoiogicai Survey .Trunci
  17. 17. Minisfry of Energy, Mines and Petmleum Resoumestreme, comprising five distinctly different breccia phases garnet. In thin section, all phases contain some, ,:enerallyand numerousdikes. The breccias differ in clast-to-matrix fine-grained, garnet.ratios, megacryst ahuudances @lack salitic pyroxenelgreen The HP pipeunique, in many respects, from the other isdiopside/biotite) and the presence or absence of additional pipes in the Golden area. Petrography and mineral chemis-phases such as oxides and spherical structures (Table 15). try suggestthat it is ultramafic lamprophyre with an ailliiticContacts between breccia phases may he gradational or affinity. There also appears to be a significant metasomaticsharp (Plate 44). A well-developed foliation, at high angles or metamorphic overprint introducing phases such asto the pipes eastern and western margins, is present in melanite/andradite and clinoamphihole.phases B1 and B2 (Plate 45). The other phases aremoder-ately to weakly foliated. GEOCHEMISTRY OF DIATREMES AND The matrix of breccia phases B1, B2 and B3 is com- RELATED DIKESposed of calcitefbiotitQcolourless clinoamphihole (tre-molite?)>chlorite. Serpentine, talc and pyrite are also Diatreme breccias and dikes in the Golden area are nl-reported to he present (Ijewliw and Schulze, 1989). Fine- 20 trahasic; silica values predominantly range from to 40%grained spinel and garnet are disseminatedthroughout. The (Table 16)with theexception dikes from theVa1f:nciennes ofgarnets are suhhedralto euhedral in outline, light green to River area which contain from to 45% silica. Analyses 40golden or brown in colour andoften optically zoned (Plate are varied with respect to the other major elemenrs (Figure46). X-ray spectra indicate that the brown garnet is melanite, 69; Table 16) and chemical classification is difficult. Altera-a titanium-bearing andradite, and the green gamet is tita- tion is intense, and may have affected some elemmts morenium-free andradite (Ijewliw, 1986, 1987). In addition to than others (e& potassium, as is suggested byth~: that fact most micas havebeen strongly altered and potassium maysmall disseminated garnets, larger garnets commonly occur have been removed). Ingeneral, analyses plot in or near thein dikes and associated with calcite segregations in clast andspherical structure-supported breccias, Groundrnass spinels alkaline to ultramafic lamprophyre fields (Wgurw 69 andare titanium-bearing magnetites. The groundmass has a 70); the HP pipe borders on alnoitic in composit:on, dikesmagmatic texture, hut apparently has been altered, eitherby from the Bush River area and from Mons Creek more fallmetamorphism or metasomatism. The matrix ofthe spheri- within the alkaline lamprophyre range anddikes from thecal structure-rich breccias is predominantly calcite with or Valenciennes River area (all strongly altered) trend towardwithout amphibole (colourless to slightly bluish). Euhedral a basaltic composition.garnets commonly form rims on the spheres. The spheres As is the case with major element compositims, traceare composed of material similar to the matrix of the other element ahundances diatremes and in dikes from the Goldenbreccia phases and are commonly cored by pyroxene or area are quite varied. There is a fairly wide range elements icmica megacrysts(Plate 47). such as chrome (Figure 7 l), strontium, rubidium and barium (Table 16).The HPpipe, in general, shows more restricted Black clinopyroxene (salite; C.E. Fipke, personal com- compositionsthan the others, has higher average rubidium,munication, 1987) bright green chrome diopside and biotite strontium and barium concentrations and has much moreare the dominant megacrysts. The black clinopyroxenesare restricted total strontiummhidium ratios (Figure72). Rocksrich in titanium (up to 2.6%) and aluminum (up 14.5%). to from the Valenciennes River and Bush River generally areasThe clinopyroxenes are compositionally similar to those show the most variation and have large ranges in total stron-found in the Isle Bizard alnoite and in South African olivine tium:ruhidium ratios, which may indicative of higher de- bemelilitites. Biotite x-ray spectra show high ironmagnesium grees of alteration. Rocks collected from BushRiver wereratios and occasional zoning to iron-rich rims. Red-brown more enriched titanium than those from elsewhere the in inchrome spinels with minor amountsof aluminum, magne- Golden area (Figure 72).sium and titanium have also been identified. The spinelshave a very limited compositional range; Fe2+/(Fe2++Mg) Although chemistry alone is not sufficient grounds tovalues of 0.34 to 0.38, Cr/(Cr+Al) values of 0.54 to 0.68 and classify rocks, especially altered rocks, it does ba2k nppet-Ti02 of less than 1%. The spinels are also compositionally rographic observations. The HP pipe appears to share manysimilar to those from Isle Bizard. Euhedral apatite phe- characteristics with ultramafic lamprophyres; the othernocrysts are also present (Ijewliw, 1987; 1,jewliw and pipes and dikes in the Golden area are slightly different, inShulze, 1988). general more similar to alkaline lamprophyres. The breccia pipe and surrounding sediments cut by are GEOCHRONOLOGYnumerous dikes (Figure 68). The dikes are generally finegrained andmassive, but are variable with respect to xeno- In the Golden area, the HP pipe and some the dikes 01lith, macrocryst,spherical structureand obviousgametcon- in the Bush River area are relatively unaltered and containtent. D l dikes are massive and free of inclusions. They abundant micas. These two locations were sampled, micacontain biotite and spinel phenocrysts; the biotites are often separates obtained and potassium-argon and ~uhidinm-aligned (flow foliation?). D2 dikes have some (less than 5% strontium analyses performed in order to establish the agetotal) small limestone xenoliths and macrocrysts; they may of the intrusions. Elsewhere, the rocks are either loo alteredalso contain minor amounts of spherical structures. D3a or too poorin micato attempt to date. Biotites from th~:HPdikes are megacryst rich (1520%); D3h dikes are also pipeyieldpotassinm-argondatesof39lf12and396flOMamegacryst rich and containspherical structnres and visible (Appendix2). Initial rubidium-strontiumanalyse; yielded a -Bulletin 88 87
  18. 18. date of 34W7 Ma; these analyses were rerun, using amoreaccurate ion-exchange technique and date of approxi- a newmately 400 Ma was obtained (Appendix 2). Close agree-ment of potassium-argon and rubidium-strontium datessuggests that the HP pipe was emplacedat approximately400 Ma. Recent paleomagnetic work (Symons and Lew-chnk, in press) established that samples fromthe HP pipe,after tilt corrections, give a concordant Mississippian pole,which is slightly younger than the isotopic age. Preliminary results of rubidium-strontium ion ex-change analyses of micas from alkaline dikes in the BushRiver area suggest an ageof 410 Ma (Appendix 2), whichis in close agreement with results from the HPpipe. Zircon separates were obtained from rocksin the Va-lenciennes River, Lens Mountain and Mons Creek In areas.all cases, the zircon populations are very heterogeneous.Zircons from the Mons Creek area vary from rounded,frosted, colourless to pale yellow spheres, to clear, equantfragments. In the Valenciennes River sampleszircons varyin shape from round to rounded prisms. Some grains arerounded andbroken. Colours range clear to colourless, fromfrosted to pink. In the Lens Mountain sample clear and both / A (NozO+K20) Figure 70.Major element ternary plots, Golden diatremes. A HP 25.00 * 4 * Mark Lorry Mons 20.00 a1noite from Alno I 1500.00 1 Averoge (Mark) u15.00 1000.00- alkali-olivine ,v E a bpsalt ! 10.00 (alkaline larnprophyre) a Average mei1titite & naphslinito ._ z bosalt 500.00- 5.00 0.00 2 m m 500.00 1000.00 1500.00 I Cr w mFigure 69. Major element discriminant diagrams, diatreme Goldenswarm. Figure 71. Ni vs Cr plot,Golden diatremes.88 GeologicalSurvey Branch
  19. 19. Ministry o Energ3 Mines and Petroleum Resources f pink roundedzircons are present, as well as clear, frosted to clear, colourless, rounded prisms. Euhedral zircons from Mons Creekyielded .1 concor- dantlead-lead ageof 469iz17 Ma (Appendix 2), which pos- sibly represents the age of zircon crystallization in the lamprophyricmagma. This may equivalent to, or slightly be older than, the actual age of emplacement. This dzte is con- sistent with the fact that the igneous rocks are hosted by Lower to Middle Cambrian strata. Rounded zircons from the same sample gave ages of 1917 to 1907 Ma(Appendix 2); these zircons are clearly xenocrystic and thedates may . be representative of the age of the basement. Zircons separated from a diatreme in the Va1,:nciennes River area are all xenocrystic in origin and gavelead-lead-00.0 ages of approximately 1525,1825,2550 and 2565 Ma (Ap- 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 pendix 2). All analyses are discordant. Zircons separated Sr/Rb from the Lens Mountain sample werealso all xlnocrysts. Resultant lead-lead ages are approximately 1790,2050 andFigure 12. SrRb vs Ti&, Golden diatremesand related dikes. 2685 Ma (Appendix 2). Bulletin 88 89
  20. 20. British Columbia - LEGEND p m H e l i k i a n Purcell Supergroup, 1 and Hadrynian Windermere Supergroup aCarnbrian aOrdovicianMckayGroup Upper Cambrian/Lower through Ordovicianand/or SilurianBeaverfoot Frn. a MiddleDevonian:Cedared, Burnais Harrogate and Fm. Middleand/orUpper Devonianthrough Mississippian m P e r r n i a na n d Pennsylvanian, Rocky Mountain Group m T r i a s s i ca n dJ u r a s s i c or younger 0 Alkalic ultrabasic diatreme ordikelocation 1 Joff Pipe 2 Rus Pipe 3 Mary Creek-White River brecciadike 4 BlackfootDiatremes 6 Swanson Peak volcanics anddiatreme (Swan Claims) 6 QuinnDiatremes 7 Summer 1 & 2 Pipes 8 CrossingCreek Kimberlite I 0 15 Kilometres Figure 73. General geology and diatreme locations Bull River - White River area. Geology modified the in from Leech(1960, 1979) and Price (1981). ~-90 Geological
  21. 21. Ministry of Energy, Mines and Petroleum Resources ULTRABASIC DIATREMESIN THE BIJLL RIVER - ELK RIVER AREA, SOUTHElRW BRITISH COLUMBIA (82Gr, 3) " Forty or more breccia pipes and related dike-rocks oc- ing fine-grained altered clinopyroxene and somefeldspar.cur within the Bull, White and Palliser river drainages (Fig- Microprobe analyses indicate that the feldspar is e:;sentiallyures 2 and 73) east of Cranbrook and Invermere (Grieve, pure albite; the albite, however, may be secondary, having1981). The majority are hosted the Ordovician-Silurian hyBeaverfoot Formation and underlying Mount andlor Wilson replaced an earlier, more calcic plagioclase or potassiumSkoki formations and exhibit similarities in petrography,de- feldspar. Some feldspar laths have been partially altered togree of alteration and morphology. white mica or clay minerals. Some mafic phenocrysts have been pseudomorphed by quartz and chlorite. Opaqile oxides (JOFFSHATCH MOUNTAIN AREA are abundant.CLAIMS) (825/11) A number of small diatremes and dikes have been re-ported (D.L. Pighin, personal communication, 1984) southof the Palliser River on the ridges around Shatch Mountain,west of Joffrey Creek (latitude 50"3107"h, longitude 115"1633W) approximately 55 kilometres east of In-vermere (Figure 73). Two were examined, both exposed atthe 2750-metreelevation and accessible by helicopter. Bothare hosted by moderately to steeply east-dipping Ordovi-cian-Silurian Beaverfoot-Brisco strata which are charac-teristically massive thick-bedded grey limestones whichcontain rugosan corals. The main breccia pipe (Figure 74) consists of small (upto 10-centimetre) subrounded to subangular fragments a instrongly foliated light green matrix. Clasts are predomi-nantly limestone, dolostone andshale; some cognate xeno-liths and rare pyroxenite nodules present. In thin section, arehoney-coloured altered vesicular glass lapilli are the pre-dominant constituents; locally the glass is completely de-vitrified. Juvenile lapilli are also present and in somesamples quite abundant. The matrix the breccia consists ofof calcite with some quartz and chlorite and minor talc,anatase and apatite. This is a tuffisitic crater-infill breccia. East (stratigraphically up-section) ofthe main breccia,intensely hematized, discontinuous layers consisting of ju-venile lapilli, suhangular lithic fragments, quartz and car-bonate are apparently interbedded with grey limestone.These agglomerate layers locally display moderate to well- ;DIMENTARY SEQUENCE INTRUSIVE AND RELATECdeveloped graded bedding (Plate 48). Elsewhere, graded SEQUENCE DOLE AND/OR UPPER DEVONIANbreccia layers are immediately overlain by pink and huff a 8 A S A L UNITpink and bulldolostones, sandy crosshedded dolostones and sandstones, Eandrlone. dolonfone. mud- stone and 101uIion brecciawell-bedded siltstones and dolomiticsiltstones of the basal ~OOV~C~AN/SILUR~ANDevonian unit (Plate 49). a 8 E lhiok Rbedded.FORMATION: AVE FOOT -Epicloolic ond/or pyro~laslic grey tuff breccia beds South of the main tuffkitic breccia, a medium to fine- forsilifemus limerfane -Folioled tuf,isilic brecc ~grained, massive igneous intrusive crops ont (Figure 74). -Marrive. magmoiis phasethat is medium to dark greenin colour withintensely hrec-ciated and hematized margins. It contains 10 to 15%altered Figure 14. Geology of theJoffpipe,ShatchMountainareaclinopyroxene and olivine phenocrysts in a matrix contain- (modified from Pell, 1986a). See text for geographic coordinates.Bulletin 88 91
  22. 22. British Columbia - Due to intensity of alteration, classification is d i f f i d t . The presence of glass lapilli and absence of biotitdphlo- gopite exclude these rocks from ultramafic lamprophyre the clan. The phenocryst and microphenocryst assemblage of olivine, two generations of clinopyroxene and a m a l l amount of feldspar (albite) in the groundmass of the por- phyrytic dike-rocks suggest that they mayshare some affin- ity to limburgites or alkaline basalts. THE RUSSELL PEAK DIATREMES (825/6) Diatremes in southern British Columbia are typified by those near Russell Peak (latitude 5Oo2540"N, longitude 115"1330"W, Figure 73). There are at least three small pipes in the immediate vicinity of Russell Peak, two more small intrusions crop out approximatelykilometres to the 7 north (latitude 5Oo2915"N, longitude 115°1540W). All of these pipes can be reached helicopter from Fairmonr.Hot by Springs. One, immediately southRussell Peak, is particu- of larly well exposed on a cliff face anddisplays many feawes of pipe morphology (Figure 75). The lower poaionof the exposed pipe comprises well-foliated, tuffisitic breccia con- taining abundant subangular fragmentssedimentaryrock ofPlate 48.Graded bedding in extrusive epiclastic layer, pipe. Joff m C 0 o r z . e contact breccia Dialreme material/tuffite a Epiclarlicand/orpyrocioslic material mixed with sediment =Mafic volcanic rock Snow patch 0Limestone rafts Db = Basol Devonian unit Ob = Ordovician Beaverfoot Formation . Otp = Ordovician TipperaryPlate 49. Epiclastic crater-infill breccia, Joff pipe, immediately Formation L - 3 Metresoverlain by well-bedded, pink and buff-weathering strata of thebasal DevonianUnit, (colourphoro, page136). Figure 75. Geology of the Russell Peak diatreme., .92 Geological Survey Branch

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